1. Field of the Invention
The invention relates to methods and apparatus for making doped diamonds and the doped diamonds made according to these methods. More particularly, the invention relates to methods and apparatus for doping a CVD diamond in order to increase the electrical conductivity of the diamond and to the diamonds produced according to these methods.
2. State of the Art
A diamond is an allotrope of carbon exhibiting a crystallographic network of exclusively covalently bonded, aliphatic sp3 hybridized carbon atoms arranged tetrahedrally with a uniform distance of 1.545 Angstroms between atoms. Due to this structure, diamonds are extremely hard and have a thermal conductivity approximately four times that of copper while being an electrical insulator. Although diamonds are most popularly known for their gemstone qualities, their hardness, thermal, and electrical properties make them very useful for many industrial applications.
Diamonds are used to true the surfaces of precision grinding wheels. In machine shops, tools tipped with diamonds cut grooves around automobile pistons and perform other precision-cutting tasks. Needles tipped with diamond dust are used to drill holes through other diamonds, which are then used as feeder nozzles for oil furnaces and as wire-drawing dies. Geologists and engineers use diamond-tipped hollow steel bits for drilling into the earth to secure samples of deep-lying rock formations. More recently, diamond films have been used in microelectronic applications as a heat sink or substrate for semiconductor devices.
Naturally occurring diamonds are believed to be the result of pure carbon having been subjected to tremendous pressure and heat deep within the earth. Synthetic, or man-made, diamonds became possible in 1955, when the General Electric Company used laboratory equipment to subject graphite to great pressure and heat. Today, diamonds can be grown as an equilibrium phase at high pressures or under metastable conditions at low pressures. One of the methods developed in recent years for producing diamonds is known as chemical vapor deposition (CVD). CVD methods use a mixture of hydrogen and a gaseous carbon compound such as methane which is activated and contacted with a substrate to produce a diamond film on the substrate. The hydrogen gas is disassociated into atomic hydrogen and then reacted with the carbon compound to form condensable carbon radicals including elemental carbon. The carbon radicals are then deposited on a substrate to form diamond. Some of the CVD methods use a hot filament, typically at temperatures of up to 2000.degree. C., to provide the thermal activation temperatures necessary to bring about the conversions described above. These methods are referred to in the art as "hot filament chemical vapor deposition"(HFCVD). Another well known CVD method is the plasma jet system. In plasma jet systems, atomic hydrogen gas is typically introduced into a plasma torch which produces a hydrogen plasma jet by means of a direct current arc, an alternating current arc, or by microwave. The plasma torch is hot enough (typically approximately 10,000.degree.K) to reduce gases to their elemental form. The torch is directed toward the substrate and the reagents exit a nozzle, or distribution head, and are deposited on the substrate.
It is generally known in the art that the physical properties of the manufactured diamond may be altered during the manufacturing process. For example, it is known in the literature that the introduction of a dopant (impurity) during the CVD process can affect the electrical characteristics of the resulting diamond. U.S. Pat. No. 5,112,775 discloses that an n-type semiconducting diamond film can be made by HFCVD using a solution of methyl alcohol, acetone, and diphosphorous pentoxide which is vaporized and used as the reactant gas with hydrogen to deposit a doped diamond film on an N-type silicon substrate. The resistivity of the phosphorous doped film was found to be approximately 10.sup.2 .OMEGA.-cm regardless of the concentration of phosphorous in the reactant gas. The literature also discloses that a boron doped diamond film can be used to make a Schottky diode which has excellent rectifying characteristics. See Zhang, X. K. et al., "Boron Doping of Diamond Films by B.sub.2 O.sub.3 Vaporization", Phys. Stat. Sol. (a) 133, 377 (1992). The resistivity of the boron doped diamond films was found to be approximately 10.degree. .OMEGA.-cm to 10.sup.-2 .OMEGA.-cm depending on the temperature of the boron at the time of the CVD process. All of the known doping methods require that a gas or an entrained powder be introduced into the reaction chamber with a flow rate controller. Various apparatus have been devised for this purpose. A typical plasma jet apparatus with a dopant feed pipe is disclosed in U.S. Pat. No. 5,260,106. The feed pipe is arranged to dispense dopant at a controlled rate to the tip of the anode.
Depending on the application, the manufactured diamond film may be deposited directly onto a substrate where it will remain, e.g. diamond may be directly deposited onto the tips of cutting tools. In other applications, a free-standing diamond film is removed from the substrate onto which it was deposited. In most of these applications it is necessary or desirable to cut, etch, or mill the free-standing diamond. State of the art operations of this type utilize a laser to cut, etch, or mill the diamond. The lasers used in these operations are relatively expensive.
Electrical discharge milling (or machining) (EDM) is an inexpensive process which has been used on many conductive materials but which has never been used successfully on a diamond film.